1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor, a power amplifier including the same, and a method for fabricating a heterojunction bipolar transistor.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2005-236259 discloses a heterojunction bipolar transistor (hereinafter referred to as an HBT in some cases) including an n-type InGaP emitter layer, an AlGaAs emitter ballast resistor layer, and a GaAs layer interposed therebetween. The AlGaAs emitter ballast resistor layer is formed by metal-organic vapor phase epitaxy and has an Al-to-As molar ratio of about 0.33, an Si concentration of about 1×1017 cm−3, and a film thickness of about 120 nm.
In general, a power amplifier uses a large amount of electricity and therefore includes a large number of HBTs (hereinafter referred to as unit HBTs), connected in parallel, having a small emitter size.
However, the following case occurs: the case where all unit HBTs connected in parallel do not operate uniformly because of unevenness due to a HBT-fabricating process or the like. In this case, currents may possibly concentrate on specifics of the unit HBTs to cause thermal runaway, leading to element breakdown.
In order to prevent the thermal runaway, a ballast resistor is generally provided in an emitter or a base for each unit HBT.
A technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-236259 is described herein in association with a mechanism for preventing thermal runaway by an AlGaAs emitter ballast resistor layer. The temperature dependence of resistivity of the AlGaAs emitter ballast resistor layer is as illustrated in FIG. 20. When a unit HBT begins to thermally run away, the temperature thereof increases due to current concentration. The emitter ballast resistance of the unit HBT beginning to thermally run away increases sharply with the increase in temperature thereof (for example, at about 100° C. or higher as illustrated in FIG. 20). The sharp increase in emitter ballast resistance thereof suppresses the emitter-base voltage of the unit HBT beginning to thermally run away and therefore suppresses the concentration of currents on the unit HBT beginning to thermally run away, thereby avoiding thermal runaway.
On the other hand, a power amplifier needs to keep power amplifier (power amplifying) characteristics not only at room temperature but also at elevated temperatures (for example, about 85° C. to 100° C.) as a whole. However, other unit HBTs that are not in thermal runaway are each equipped with an emitter ballast resistor. As is clear from FIG. 20, the emitter ballast resistance of an HBT increases by about 13% or about 21% at, for example, about 85° C. or about 100° C., respectively, as compared to the emitter ballast resistance at room temperature, that is, the parasitic resistance thereof increases.
Therefore, in the case of fabricating a power amplifier by the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-236259, the thermal runaway of HBTs can be prevented. However, there is a problem in that the increase in emitter ballast resistance due to an increase in temperature deteriorates power amplifier characteristics at elevated temperatures.